What heater size for a 38 gallon tank?
36 × 12 × 20 in · freshwater planning
38 gallon · heater wattage
The working
38 nominal gal × 3 W/gal = 114 W for a mild rise; × 5 = 190 W for a larger rise. At a 10°F rise the rounded choice is 1 × 150 W.
| Temperature rise | Estimated W | Common units / installed W |
|---|---|---|
| 5°F | 114 | 1 × 150 / 150 |
| 10°F | 114 | 1 × 150 / 150 |
| 12°F | 152 | 1 × 200 / 200 |
| 15°F | 190 | 1 × 200 / 200 |
| 20°F | 190 | 1 × 200 / 200 |
First decisions for this medium setup
Before choosing power for 38 gallons, decide where the unit fits with the filter intake and substrate. A horizontal or diagonal installation requires explicit manufacturer permission. During a water change, the waterline can expose the upper part of a vertically mounted heater; unplug it and observe the maker’s cooling instructions.
38 gallon equipment and layout plan
This upright rectangle adds vertical room without a wider bottom. The 36 × 12-inch base has an aspect ratio of 3:1. A lid or light must span roughly 36 inches; measure rim openings and bracket adjustment before ordering.
| Room-to-target rise | Calculated need | One-unit option | Two-unit option |
|---|---|---|---|
| 10°F | 114 W | 150 W | 75 + 75 W |
| 15°F | 190 W | 200 W | 100 + 100 W |
| 20°F | 190 W | 200 W | 100 + 100 W |
One correctly sized unit keeps installation simple. Two smaller thermostatic units spread heat, but do not make a stuck-on thermostat safe; check temperature independently. The 36-inch span determines how far heat must circulate; the 20-inch height also limits vertical mounting clearance.
Moving up from the 37 gallon changes watts of calculated demand at a 20°F rise from 185 to 190: an increase of 2.7%. Budget for the larger requirement rather than reusing the smaller plan unchanged.
Assumptions and limits
These bands (3 W/gal up to 10°F, 4 between 10 and 15°F, 5 at 15–20°F) are estimates, not manufacturer guarantees. Covers, drafts and evaporation matter. For rises above 20°F, use the maker’s sizing chart. Split heating above 55 gallons; verify temperature with a separate thermometer. Unplug heaters before exposing them during maintenance.
Adjust measurements with the heater wattage calculator
Questions before you buy
What dimensions does this 38 gallon calculation use?
It uses a rounded 36 × 12 × 20 inch rectangle. Actual dimensions vary by brand and frame; measure your own tank.
What is the heater wattage estimate for a 38 gallon tank?
114–190 W planning range
Equipment to check
Compare adjustable aquarium heater. Check dimensions, capacity and product instructions before buying.
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